1. Field of the Invention
The present invention relates to an apparatus for controlling a tuning point by a tuning voltage which is applied to a tuning circuit and relates to a tuning circuit having a function of controlling a tuning point.
2. Description of the Prior Art
Generally, a tuner includes a tuning circuit, a mixer and an oscillator. The tuning circuit is tuned to a radio frequency signal of a bandwidth which is selected by a user among a plurality of broadcasting signals which are received via an antenna, and outputs the tuned radio frequency signal to the mixer. The oscillator generates a continuous wave which is equivalent to the radio frequency signal which is tuned by the tuning circuit. The mixer mixes the tuned radio frequency signal from the tuning circuit with the continuous wave signal from the oscillator, and outputs an intermediate frequency signal. Most of the tuning circuits have a function of varying a tuning frequency by using a varactor diode in order to be tuned to a specific bandwidth of the radio frequency signal. In the tuning circuit which includes the varactor diode, the tuning frequency is tuned by applying a tuning voltage which is equivalent to a specific frequency to the varactor diode. The tuning voltage which is applied to the tuning circuit is discretely distributed on the basis of assigned channels. Thus, in order to change the tuning frequency of the tuning circuit, i.e., the channel of the tuner, the tuning circuit can be tuned to each of the channel frequencies by discretely providing the tuning voltage to both terminals of the varactor diode of the tuning circuit.
FIG. 1 is a circuit diagram for showing the circuit configuration of a conventional tuning circuit for a tuner. As shown in FIG. 1, a tuning circuit 10 comprises a varactor diode VD1, an inductor L1 and a first capacitor C1. Varactor diode VD1 is connected in parallel to both inductor L1 and first capacitor C1 in order to configure a bandwidth filter. Varactor diode VD1 is connected to an external voltage source via a lead wire 102 so that a voltage is applied from the external voltage source to varactor diode VD1. When the voltage is applied to varactor diode VD1 via both terminals thereof, the applied voltage changes the capacitance of varactor diode VD1, thereby changing the tuning frequency of tuning circuit 10. Inductor L1 is connected in parallel to varactor diode VD1, and a third capacitor C3 is connected in series to inductor L1. Both second and third capacitors C2 and C3 are transmitting capacitors, and both fourth and fifth capacitors C4 and C5 are capacitors for removing an alternating current signal.
Therefore, working for tuning points is generally required in manufacturing tuners. The working for tuning points is performed by applying the tuning voltage which is equivalent to the specific broadcasting signal to a terminal 104 and by confirming whether or not tuning circuit 10 is accurately tuned to a relevant frequency signal according to the applied voltage. The work for tuning points is a process for confirming whether or not the tuning circuit is accurately tuned by a specific voltage which is applied to varactor diode VD1 of tuning circuit 10. When the tuning circuit has an error with respect to the applied voltage, a tuning frequency of the tuning circuit is adjusted by varying an inductance of inductor L1.
Therefore, the inductance of the inductor should be manually adjusted in manufacturing these tuning circuits and therefore, the productivity of the tuning circuits decreases by the number of the work for tuning points. Also, although the relevant tuning frequency is adjusted to coincide with the tuning voltage, the tuning point is still unstable because of the influence of an environment in practical use.
In U.S. Pat. No. 3,867,568 issued to Kenneth A. Merriweather on Feb. 18, 1975, an apparatus for controlling a tuning point for a tuning circuit is disclosed. A controlling circuit for an automatic frequency control system according to this patent comprises a synchronizing separating section for detecting a regenerative synchronizing pulse, a detecting section connected to the synchronizing separating section and for detecting the absence of the detected synchronizing pulse in a specific frequency transmitting bandwidth, and an automatic frequency control cut-off section for effectively disabling an automatic frequency control circuit for varying the tuning frequency in accordance with the absence of the detected synchronizing pulse. According to the description in the U.S. patent, an inductor should be manually adjusted in manufacturing the tuning circuits and thereby the productivity of the tuning circuit decreases by the number of the work for tuning points. Also, although the relevant tuning frequency of the tuning circuit is adjusted to coincide with the channel voltage, the relevant tuning point is still unstable because of the influence of the environment in practical use.